Steven A. Bradley

Composition and microstructure of silicon carbide whiskers

The bulk and surface chemistries of four sets of commercially available SiC whiskers made by three manufacturers were determined. The oxygen content varied significantly, ranging in the bulk from 1.9 to 0.6 at.% and on the surface from 35 to 15 at.%. Surface analysis as obtained by X-ray photoelectron spectroscopy also indicated that the oxygen species differed significantly with whisker supplier; each of three of the whisker sets contained a surface species that is very similar to that found in a Si-O-C glass, while one whisker surface appeared to have a silica-rich surface. Surface carbon concentrations varied significantly, while silicon concentrations did not. Scanning transmission electron micrographs indicate significant morphological variations (i.e., twinning, branching, kinks, surface roughness, etc.) occur in all of the whisker types.

CHARACTERIZATION OF RECENT SILICON CARBIDE WHISKERS

SiC whisker surface chemistry and morphology can strongly impact composite processing and properties. We report here the surface chemistry and morphology of SiC whiskers received during late 1987 and 1988 from five sources. Comparisons are made with previously characterized whiskers.

Analysis of grain boundaries for reaction-bonded silicon nitride with yttria addition

Silicon nitride is often the developmental ceramic of choice for high-temperature engine components. Sintering this ceramic, however, is very difficult due to the covalent nature of the bonding. Complicating the matter, Si3N 4 tends to dissociate at temperatures above 1700 ° C. Dissociation can be impeded by application of a nitrogen overpressure, while the general sintering problem is overcome by the addition of small amounts of various oxides. These sintering aids react with the silicon nitride and each other to form liquid phases, allowing sintering at temperatures between 1600 and 1900°C. Common additives include magnesia, yttria, alumina, and their various combinations. Normal additive content is in the range of 1 to 10% by weight. Although these sintering aids produce highly dense structures, they also lead to the formation of secondary phases along grain boundaries. The resulting materials are very strong at room temperature, but they rapidly lose their strength at high temperature (> 1000°C) because high-temperature mechanical properties such as creep are controlled by the grain boundary phases. The grain-boundary phases, in turn, are determined by the quantity and combination of sintering aids, by the firing cycle, and by the densification process (e.g. hot-pressed or reaction-bonded). Thus, by correlating the microstructure with the physical properties and processing/composition with microstructure, the effect of processing and compositional factors upon the physical properties can be determined and controlled. Although MgO was initially used as a sintering aid in the past, Y203 and YaO3-A1203 yield greater hightemperature strength [1, 2]. Yttria is an attractive additive because at low temperatures it reacts with the SiO2 (and some Si3N4) on the surface of the Si3N4 to form a liquid, greatly enhancing the sintering rate [3]. At higher temperatures when densification is complete, the liquid reacts with more Si 3 N 4 to give a highly refractory bonding phase along the grain boundaries. Tsuge et al. [4] showed that crystallization of the grain-boundary phase could improve high-temperature strength. Investigations of Si3N4-Y203 and Si3Na-Y203SiO2 phase diagrams [3, 5, 6] have been used to predict and explain the various grain-boundary constitutents and resulting properties. Using hot-pressed materials, Lange and co-workers [5, 7] found that the phases within the Si3N4 Si2N20 Y2Si207 compatibility triangle (in the Si3N4 Y203 SiO2 phase diagram) were extremely oxidation resistant, while Si3Y203N4, YSiOzN (K-phase), YxoSiyO23N4 (H-phase) and Y48i207N2 (J-phase) readily oxidize at 1000 ° C. Richer-

Examination of the high-temperature (850° C) oxidation of an Ni-Si-B powder

An air-fired (850° C) alloy powder of Ni-Si-B has been prepared to test its suitability in silk screen conductor applications. The silicon and boron were included because, when alloyed with nickel, they greatly impede the oxidation of the latter, removing the necessity for inert gas firing. The physical aspects of this novel material have been examined by SEM, while its chemical nature was studied by ESCA. It was found that during airfiring the alloy powder forms a glass around the conductive metal particles impeding the oxidation of nickel. Preferential migration of boron and silicon to the surface of the powder is indicated. Speculations are presented for the conductive pathway for this material.